With the increased acceptance of and demand for electronic display and information systems, as in automobiles, systems designers have been confronted with a variety of problems. Some of these problems relate to providing data output in a form most useful to viewer, and others relate simply to asthetic issues. One problem in particular, however, has presented a more serious issue because it touches upon both state and federal legal restrictions and requirements; the provision of an electronic odometer.
The task of the vehicle odometer can be described easily enough. The odometer must accurately measure and report the total distance the monitored vehicle has ever been driven. This information will be relied upon by maintenance personnel and by potential purchasers of the vehicle. An odometer that fails to accurately measure travelled distance, or that fails to accurately report this distance, can contribute to incorrect decisions by these persons.
Current mechanical odometers meet these needs by providing geared members that turn in accordance with a calibrated schedule that relates to the distance being travelled by the vehicle. Simply stated, when the vehicle moves, the gears turn and the odometer count advances. When the vehicle does not move, the gears do not move and the odometer readout remains quiescent though viewable.
Various solutions have been provided to allow the distance travelled by a vehicle to be electronically monitored. Similarly, a variety of display mechanisms are available to allow the odometer reading to be viewed. The problem with providing an electronic odometer, however, centers upon the memory mechanism. More particularly, many electronic memory units require the continued presence of electricity in order to support their storage function. Upon removing such electricity, the contents in memory are lost. Although such memory units can be supported by the vehicle battery, such batteries occasionally run down or must be disconnected to be replaced. At such time, the memory contents would be lost.
Some memory units, such as read only memory units (ROM's) are not susceptible to such a loss of memory upon disconnection of power. Unfortunately, such memory units cannot have information input into them except during the manufacturing process. Such memory units would therefore have obvious drawbacks in a odometer setting.
There are other memory units, known as EEPROMS (electrically erasable programmable read only memory), that provide non-volatile memory and that avoid these problems. These memory units will retain their contents in memory even in the absence of electric power. Further, these memory units can have specific memory locations selectively erased and re-written to during use by simply providing appropriate signals from a control unit, such as a microprocessor.
Unfortunately, even EEPROMS have a problem associated with their use; they can wear out. More particularly, each memory location has an expected erase/write cycle lifetime of 10,000 erase/write events. Since an odometer memory must have information regarding distance travelled entered into it on a highly regular basis (to ensure accuracy), the number of anticipated erase/write cycles can surpass the expected lifetime of the memory unit.
To compensate for this, some prior art systems use EEPROMS for storing information regarding the most significant bits of a particular count, and volatile memory for storing information regarding the least significant bits. By this compromise, EEPROM usage can be minimized somewhat to acceptable levels. Unfortunately, least significant bits data can be lost from time to time, and this can contribute to inaccuracy.
Other prior art systems have improved this technique by employing circuitry to detect a power down condition, at which time additional supporting circuitry provides an additional 30 milliseconds or so of power so that the least significant bits can be transferred to the EEPROM for non-volatile storage. Unfortunately, the cost of the extraneous hardware for detecting and supplying power is not tolerable in many applications.
There therefore exists a need for a non-volatile memory storage system that provides a non-volatile memory that may be satisfactorily utilized in an odometer setting, and that will meet specified needs for accuracy and anticipated lifetime.